Photovoltaic (PV) solar energy production depends on the global irradiance available, which depends, for a determined location, on the day of the year and the time of the day, but also on meteorological effects such as clouds or aerosols in the particular location of the photovoltaic panels.
Therefore, along any given day, the photovoltaic solar power will vary importantly. On a sunny day, for example, it will show a predictable variation because of the different levels of irradiance along the day, and because of the different angles the sun will form during the day with the photovoltaic solar panels as a function of the type of tracking capacity they have.
On a cloudy day, the production will show additional variations that can be much faster depending on the photovoltaic plant size and the speed to which the clouds move over it, wherein the power curves will be completely different for a clear day, a partly cloudy day and a completely cloudy day.
Cloudiness is difficult to forecast and fast with respect to its effects, so it can cause fluctuations in the photovoltaic plants production which cause problems in the electrical system stability. The electrical grid operators have carried out several researches about the fluctuations effects since, as they cannot be foreseen, the grid has to be provided with enough control capacity to absorb them. The maximum power variations in photovoltaic plants may even reach 90% of nominal power in very short time intervals, of less than a minute.
In the case of a wind farm, the wind resource obtained is also variable depending on the meteorological conditions, so the effects are similar to those present in photovoltaic plants, although with different dynamics and time schedules.
One of the ways to solve power fluctuations is to control the generation by limiting the power variation maximum speed, typically with the maximum ramp value which power variation may have in each control cycle. That requires predicting fluctuations and acting in advance limiting the production of the plant. In order to predict fluctuations, it would be required to accurately assess the modifications of the meteorological parameters causing them. Poor forecast of the meteorological variables and the effects thereof on the plant production may cause great losses in this process and may justify the investment in energy storage systems.
The way the established regulations deal with this problem is by setting power variation maximum ramps to the power being fed in the grid by the intermittent generation plants. In this way, it can be ensured that the power variation of a plant, or a group of plants, does not exceed the dynamics with which other plants in the system may increase or reduce power, so that the production and consumption balance is not altered at each moment.
In order to set these values, possible power variation speed for thermal power plants, between 2.5%-10% of its nominal power per minute, being part of the manageable generation, is usually taken as a reference. Thereby, it is ensured that the rest of the system, if it is provided with enough control capacity, may respond to quick power variations in the intermittent power generation plants. Another option is to consider that the aggregation of nearby plants will produce a variation in the power obtained as a sum of all of them, which is less than the variations from each individual plant (filtering effect), so it is possible to reduce the store requirements.
The plants with energy storage may control variation speed of the output by means of energy charging and discharging of the storage systems. For example, a 1.2 MWp plant can be added a 1 MW-560 kWh battery system with which output power variations from the plant can be controlled according to a maximum ramp determined by the control system.
There are different strategies known from the state of the art used in solar and wind generation plants and having several energy storage technologies, where the usual way to control the power fluctuations is by storing the excess in the storage or transferring the deficit from the storage, so that when power increments are produced, the battery is charged so that the production being fed to the grid do not have great oscillations, whereas when power drops occur, the battery provides the power to keep up with the production being fed to the grid without great oscillations, where this procedure can be carried put in different ways obtaining different results.
The immediate way of doing this, referred to as ramp-rate control, is with a control algorithm which, in the time cycle being defined, sets a charging or discharging value for the storage system, so that in the next cycle there is no value with a deviation higher than the one allowed by the maximum allowable ramp-rate value, implying lower cycling degradation.
Publication “Storage requirements for PV power ramp-rate control. Sol. Energy 99, 28-35 from the authors Marcos, J., Storkël, O., Marroyo, L., Garcia, M., Lorenzo, E., 2014”, discloses a ramp-rate control strategy that complies with a given allowable ramp-rate variation. In particular, an equation was given to calculate the storage capacity required to support the worst case fluctuation at a photovoltaic plant. In other words, a fluctuation when the photovoltaic plant is in full operation in clear sky conditions compared to completely cloudy conditions, or vice versa. As the sign of the first fluctuation is unknown, a double capacity battery is required to absorb both the upward and downward fluctuations based on currently-available knowledge.